Ceramic resistor materials such as doped barium titanates of positive temperature coefficient of resistivity have been used or proposed for use in a wide variety of applications as self-regulating electrical resistance heaters. A resistor element made from such a ceramic material normally displays relatively low electrical resistance at room temperature, for example, and then displays a small increase in resistivity as the element is initially heated by directing electrical current through the element. However, when the element has been self-heated in this manner to a selected temperature which is characteristic of the ceramic material, the element displays a very large and sharp increase in resistance and reduces current through the element to a very low level which is just sufficient to maintain the element at an equilibrium temperature. In this way, the resistance element provides a substantial amount of heat but is self-regulating to avoid excessive overheating of the resistance element.
Such ceramic resistor elements are commonly used in thin disc-like form. The broad flat surfaces of the disc element are then provided with very thin metallized coatings or the like so that the broad element surfaces serve as contact surfaces for directing electrical current through all parts of the ceramic material in the element. In this way, all parts of the ceramic element material tend to be heated substantially simultaneously to the selected temperature at which the ceramic material displays its sharp increase in resistivity. However, in this arrangement, the broad contact surfaces of the resistor element also constitute the principle heat-emitting surfaces of the element. Accordingly, the means commonly employed for making electrical connection to the contact surfaces of the resistor element can retard emission of heat from these element surfaces. Further, the means used in making electrical connection to these element surfaces have frequently tended to cause degradation of the electrical properties of the ceramic constituents of the resistor element. These factors have tended to result in a significantly reduced total heat output from a heater device utilizing such a ceramic resistor element and have frequently caused the heater device to have a relatively short service life. For example, where the broad contact surfaces of such a ceramic resistor element are secured to heater device terminals by the use of electrically conducting adhesives, the layers of adhesive formed on the resistor surfaces can have a thermal barrier effect for retarding heat emission from the resistor element. More important, such adhesives have tended to cause degradation of the resistance properties of the element materials. Similarly, where terminals have been soldered to the contact surfaces of the resistor element, the fluxes used in making such solder connections have also tended to cause degradation of the ceramic materials of the resistor element. Alternately, where metal terminals are resiliently held in engagement with the contact surfaces of the resistor element in a construction which is both economical and convenient to assemble, it is frequently found that either poor electrical engagement is achieved between the terminal and the element or there is poor heat transfer from the element to the terminal so that heat emission from the resistor element is considerably retarded.
It is an object of this invention to provide a novel and improved electrical resistance heater device; to provide such a device which is characterized by a long service life; to provide such a device which is of convenient and economical construction; to provide such a device utilizing a ceramic resistor element of positive temperature coefficient of resistivity wherein good electrical engagement with the resistor element is conveniently obtained with improved heat emission from the resistor element; and to provide such an improved heater device which is compact, rugged and reliable in use.